Minutes after I posted my article today about radiation from the damaged Fukushima Daiichi nuclear power plant not reaching the United States in harmful amounts, I heard a news report about iodine-131 from the plant being detected in rainwater in Massachusetts.

Iodine-131 is a radioactive form of iodine. It’s a byproduct of the reaction that creates energy in a nuclear power plant. The Fukushima power plant has been emitting iodine-131 into the atmosphere.

Both are helpful and address issues ranging from the risk to pets who drink rainwater (“unlikely to harm your pet”)” to kids playing in the rain (current levels are “far below those of public health concern”) and switching to bottled water (“no need” at this time).

Here are some key points about the latest news:

The iodine-131 was found in a rainwater sample, not in drinking water. When Massachusetts officials tested water from two reservoirs used for drinking water, they didn’t find detectable levels of iodine-131 contamination from Japan.

The iodine-131–contaminated rainwater sample had a radiation reading of 79 picoCuries per liter, according to information posted by Massachusetts health officials. That is over the EPA’s limit of 3 picoCuries per liter for drinking water. But the EPA’s FAQ says the agency’s drinking water limit was based on calculations that presume a lifetime of exposure. The EPA says the levels of radiation being now seen in rainwater are 25 times below the levels of concern for short-term exposure, even for infants, pregnant women, and women who are breastfeeding.

If cows eat grass and other feed contaminated with iodine-131, the radioactive particles can get into milk. Milk was the main route of exposure after the 1986 Chernobyl nuclear power plant accident. The Massachusetts FAQ says that in initial testing the EPA hasn’t found iodine-131 in milk products in the United States. The EPA is stepping up efforts to test milk for iodine-131.

Massachusetts officials say that until the Fukushima plant is brought under control, iodine-131 may continue to be detected in rainwater in the state but at levels significantly lower than any health concern.

After hearing the news about iodine-131 in the rain water, I spoke with Dr. Richard Zane, vice chair of the emergency department at Harvard-affiliated Brigham and Women’s Hospital and medical director for emergency preparedness for Partners Healthcare in Boston. My previous post today includes a question-and-answer with Zane. Here is what Dr. Zane said:

It is rainwater that is contaminated with iodine-131. When it gets mixed with groundwater, the levels will be so diluted that it will undetectable. Does it pose a health hazard? No. Does it pose a hazard if you get rained on? No. There is no reason for people to do anything [in reaction], and that includes taking potassium iodide.